Hillary Clinton’s choice of first-term Virginia Senator Tim Kaine as her running mate is probably the strongest she could have made, at least from what we know of him so far.

As with Mike Pence on the Republican side, it’s unlikely anyone will challenge his readiness to be president, given his experience in Congress and as a former governor.

Some liberals have spoken against Kaine, but he has been a mainstream liberal in the Senate. He isn't a firebrand in the mold of Bernie Sanders, Elizabeth Warren or Sherrod Brown. Instead, he occupies roughly the same ideological spot as Clinton: neither on the left wing or the moderate side of a liberal party.

But it makes little difference whether some groups in the party are enthusiastic about a running mate or not. Conventions -- at least normal ones -- unify the party faithful around the presidential nominee's choice, and supply reasons to fall in love with both parts of their ticket.

Since there’s not much of an upside with vice-presidential picks, the key is to avoid someone who creates trouble in the campaign or once elected.

No foolproof formula exists for this. In the past, the running mates who turned out to be problems, most recently Sarah Palin in 2008, had never run for president themselves. Those who had did fine: Joe Biden; Jack Kemp, who was Bob Dole's veep choice in 1996; Al Gore in 1992, and George H.W. Bush in 1980.

Donald Trump had limited choices because so few big-name Republicans were willing to take the job. Hillary Clinton had more options, but she didn't have any experienced presidential candidates to select from. Almost every Democrat who has run for president enough to be vetted by the process was either over 70 (such as Bernie Sanders), was toxic (such as John Edwards) or was unavailable (Gore).

Nevertheless, Kaine, like Pence, appears to be as likely as anyone never subjected to that national-level grilling to get through the campaign scandal free and to emerge as a capable vice president.

And if running mates help a bit in their home states (political scientists who study the issue disagree on that one), then he’s an even better pick. The FiveThirtyEight prediction model has Virginia as the fourth-most-likely “tipping point” state -- that is, the state that would determine a very close Electoral College vote. If Kaine gives Democrats even a very small boost in a critical state, then he will have proved himself to be an excellent choice, not just a safe one.

It’s difficult to prove that the problem picks cost the ticket votes in November, but it would seem damaging if the presidential nominee has to spend considerable time defending the running mate.

The others I'd count as problem picks are Richard Nixon in 1952, Spiro Agnew in 1968, Tom Eagleton in 1972, Geraldine Ferraro in 1984 and Dan Quayle in 1988.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.